Electromagnetic geophysical surveying of the Earth's subsurface involves imparting an electric field or a magnetic field into subsurface Earth formations, such formations being below the sea floor in marine surveys, and measuring electric and/or magnetic fields by measuring voltage differences induced in electrodes, antennas and/or interrogating magnetometers disposed at the Earth's surface, or on or above the sea floor. The electric and/or magnetic fields are induced in response to the electric field and/or magnetic field imparted into the Earth's subsurface, and inferences about the spatial distribution of conductivity of the Earth's subsurface are made from recordings of the induced electric and/or magnetic fields.
Often, electromagnetic surveying includes imparting a substantially continuous, time varying electromagnetic field into the subsurface formations by passing time varying electric current through a transmitter antenna. The alternating current may have one or more selected discrete frequencies. Such surveying is known as frequency domain surveying. Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient electromagnetic surveying. Therein, direct current (DC) electric current passes through a transmitter at the Earth's surface (or near the sea floor). At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface or water surface. Alternative switching techniques are possible.
The above methods have been adapted for use in marine environments. Cable-based sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. Systems with towed electromagnetic receivers have also been devised.
Note that the figures provided herewith are not necessarily to scale. They are provided for purposes of illustration to ease in the understanding of the presently-disclosed invention.